Printing techniques such as screen printing, flexography, gravure, and offset lithography have been widely used throughout history for both graphics arts and, more recently, materials assembly. However, the push towards smaller feature sizes has focused attention on other print-based approaches, such as inkjet printing1 and direct ink writing (DIW),2-4 where ink is deposited through fine nozzles onto a substrate. Inkjet printing is a droplet-based approach primarily limited to low aspect-ratio features (for example, 100 nm thick and 100 μm wide) geometries for single pass and supported features in multipass printing due to the diluted solids content of the liquid-like inks. Alternatively, DIW is a filament-based approach that affords the creation of continuous, fine scale, high aspect ratio, and self-supporting electrodes due to the high concentration and stiffness of silver particle inks expressly engineered for these properties.
The design of metallic inks for self-supporting printed electronics poses considerable challenges. Several requirements must be met simultaneously, including careful control over particle size and solids loading, appropriate rheological behavior to facilitate deposition, low resistivity at modest curing temperature, and compatibility with a variety of substrate materials. While some commercial metallic pastes (or paints) are conductive under ambient conditions, they typically contain large particles (0.1˜10 μm) and, hence, are not suitable for patterning at the microscale due to nozzle clogging.
To date, solution based synthetic methods have yielded stable silver dispersions with relatively low solids loading (typically ˜10 wt %) that are not suitable for printing self-supporting electrodes. Typically, silver nanoparticles are synthesized in solution via the reduction of silver precursors in the presence of surface capping and reducing agents. The surface capping agents usually contain functional groups, such as thiol (˜SH), carboxyl (˜COOH), or amine (˜NH) groups, whereas sodium borohydride (or citrate), hydrazine, or polyol are used as reducing agents. It is challenging to synthesize stable and highly concentrated (>50 wt %) nanoparticle inks due to their strong tendency to agglomerate.
PAA, a water-soluble polyelectrolyte is a well-known effective capping agent for stabilizing silver particles. Hydrophilic silver particles are produced by this method. The carboxyl (—COOH) group in PAA provides an active site for capping in the presence of reducing agents, such as NaBH4, H2NNH2, and light irradiation. B. H. Ryu, et al. demonstrated silver nanosol with high silver content (50 wt %) using an aqueous AgNO3/PAA-Na salt/NaBH4 system.5 X. Xu et al. reported metal nanocomposites using an aqueous AgNO3/Acrylic acid/γ-ray system.6 N. Toshima, et al. investigated silver nanoclusters using an aqueous AgClO4/PAA-Na salt/UV-ray system.7 
Other carboxyl (—COOH) group functionalized compounds have also been used as capping agents. S. Magdassi, et al. demonstrated an aqueous silver nanocolloids stabilized by carboxylmethyl cellulose as a capping agent in the presence of trisodium citrate as a reducing agent.8 In contrast, long-chain carboxylates provide hydrophobic silver particles that are dispersible in toluene or hexane.9-12 
Alkyl amines have commonly been used as reducing agents (short-chain amines), as well as capping agents (long chain amines). The silver particles obtained from this method are generally hydrophobic and only dispersible in organic solvents.13-16 
Direct printing of self-supporting electrodes, such as spanning and three-dimensional (3-D) structures is challenging. The conventional ink-jet printing method uses droplet based printing, which is not suitable for producing self-supporting electrodes due to a spreading issue of liquid-like inks. S. B. Fuller, et al. demonstrated 3-D silver electrodes by ink-jet printing using reiteration of printing and curing process for each layer.17 